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United States Patent |
5,251,683
|
Backer
|
October 12, 1993
|
Method of making a cylinder head or other article with cast in-situ
ceramic tubes
Abstract
A method is disclosed for making an article, such as a cylinder head,
having one or more hollow ceramic tubes, such as ceramic exhaust port
liners, cast in-situ therein with reduced cracking or breakage of the
ceramic tube from tensile and compressive stresses exerted on the ceramic
tube during the casting process. The method involves forming a removable
core in the ceramic tube of a core material having a thermal expansion
coefficient not exceeding about 10 times that of the ceramic tube to
minimize crack-causing differential thermal expansion-induced tensile
stresses on the ceramic tube when molten metal is cast therearound,
casting the molten metal about the cored ceramic tube in a mold cavity,
stress relieving the cast article at an elevated temperature before the
cast article cools to a lower temperature at which crack-causing
differential thermal contraction-induced compressive stresses are exerted
on the cast in-situ ceramic tube, and removing the core from the cast
article.
Inventors:
|
Backer; Gerald P. (Southfield, MI)
|
Assignee:
|
General Motors Corporation (Detroit, MI)
|
Appl. No.:
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667922 |
Filed:
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March 11, 1991 |
Current U.S. Class: |
164/98; 164/11; 164/76.1; 164/112 |
Intern'l Class: |
B22D 019/00 |
Field of Search: |
164/91,98,112,11,28,30,31,465,76.1
|
References Cited
U.S. Patent Documents
3173451 | Mar., 1965 | Slayter | 164/98.
|
3568723 | Mar., 1971 | Sowards | 164/98.
|
3825055 | Jul., 1974 | Mino et al. | 164/132.
|
3919755 | Nov., 1975 | Kaneko et al. | 29/156.
|
4003422 | Jan., 1977 | Schramm et al. | 164/9.
|
4148352 | Apr., 1979 | Sensui et al. | 164/112.
|
4346556 | Aug., 1982 | Rice et al. | 60/272.
|
4637110 | Jan., 1987 | Yamagata | 29/156.
|
4828009 | May., 1989 | Taniguchi | 164/98.
|
4840219 | Jun., 1989 | Foreman | 164/369.
|
4849266 | Jul., 1989 | Dwivedi | 29/527.
|
4938802 | Jul., 1990 | Noll | 164/364.
|
4997024 | Mar., 1991 | Cole | 164/98.
|
5012853 | May., 1991 | Bihlmaier | 164/98.
|
Foreign Patent Documents |
58-37277 | Aug., 1983 | JP | 164/91.
|
60-216967 | Oct., 1985 | JP | 164/98.
|
Primary Examiner: Rosenbaum; Mark
Assistant Examiner: Pelto; Rex E.
Attorney, Agent or Firm: Grove; George A.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A method of making a cast iron article comprising a tubular passage with
a cast-in place ceramic tube lining the passage, comprising:
(a) forming a removable core in the ceramic tube of a core material having
a thermal expansion coefficient not exceeding about 10 times the thermal
expansion coefficient of the ceramic tube so as to minimize cracking of
the ceramic tube from differential thermal expansion-induced tensile
stresses when molten iron is cast about the ceramic tube,
(b) casting molten iron about the cored ceramic tube positioned in an
article-shaped mold cavity to form a solidified, cast article with the
ceramic tube cast in-situ therein,
(c) subjecting the cast iron article and ceramic tube to an elevated
temperature stress-relieving treatment before the cast iron article cools
below about 500.degree. F., and
(d) removing the core from the cast in-situ ceramic tube.
2. The method of claim 1 including making the core of said core material
and a thermally decomposable core binder.
3. The method of claim 2 including forming the mold cavity by shaping a
mixture of molding material and a thermally decomposable mold binder.
4. The method of claim 1 wherein the step of stress relieving the cast iron
article in the mold cavity as in step (c) occurs at a temperature greater
than about 1050.degree. F.
5. The method of claim 1 wherein the core material exhibits a thermal
expansion coefficient not exceeding about five times the thermal expansion
coefficient of the ceramic tube.
6. The method of claim 5 wherein the core material is zircon particulate.
7. A method of making a cast iron cylinder head having a hollow ceramic
exhaust port liner cast in-situ therein, comprising:
(a) forming a removable core in the ceramic exhaust port liner of a core
material having a thermal expansion coefficient not exceeding about 10
times the thermal expansion coefficient of the ceramic exhaust port liner
so as to minimize cracking thereof from differential thermal
expansion-induced tensile stresses when molten metal is subsequently cast
about the ceramic exhaust port liner,
(b) casting molten iron about the cored ceramic exhaust port liner in a
cylinder head-shaped mold cavity to form a solidified, cast iron cylinder
head with the ceramic exhaust port liner cast in-situ therein,
(c) subjecting the cast iron cylinder head to a stress-relieving treatment
at a temperature of greater than about 1050.degree. F. before the cast
cylinder head cools below about 500.degree. F., and
(d) removing the core from the cast in-situ exhaust port liner after the
cast cylinder head is stress relieved.
8. The method of claim 7 wherein the ceramic exhaust port liner comprises
aluminum titanate.
9. The method of claim 7 or 8 wherein the core material comprises zircon
particulate.
10. The method of claim 7 including making the core of said core material
and a thermally decomposable core binder.
11. The method of claim 10 including forming the mold cavity by shaping a
mixture of molding material and a thermally decomposable mold binder.
12. The method of claim 11 including stress relieving the cylinder head in
the mold cavity to thermally decompose the core binder and mold binder in
step (c) to facilitate removal of the core and mold material in step (d).
13. The method of claim 7 wherein the cast cylinder head is stress relieved
at a temperature greater than about 1100.degree. F.
Description
FIELD OF THE INVENTION
The present invention relates to a method of casting an article, such as a
cylinder head, having one or more hollow ceramic tubes, such as exhaust
port liners, cast in-situ therein in such a manner as to reduce breakage
of the tubes from tensile and compressive stresses exerted on the tubes
during the casting process.
BACKGROUND OF THE INVENTION
It is known to use ceramic exhaust port liners in the exhaust channels of
cylinder heads for internal combustion engines, especially for engines
equipped with a turbocharger. The exhaust port liners minimize loss of
heat from the exhaust gases and thereby provide the turbocharger with
hotter gases to enhance its operation.
It is also known to line exhaust manifolds, pipes and mufflers with a
ceramic liner to retain the high temperature of the exhaust gases longer
for improved secondary combustion of potentially polluting hydrocarbons
present in the exhaust gases.
A typical approach to the manufacture of products such as cylinder heads
and exhaust manifolds having ceramic liners therein has involved
fabricating a hollow liner of a suitable ceramic material, filling the
hollow liner with a mixture of foundry sand (e.g., silica sand) and a
resin or other binder which cures or hardens to form a bonded core in the
liner, placing one or more of the cored liners in a mold cavity of a
casting mold (e.g., a permanent mold or a sand mold), casting molten iron
or aluminum into the mold cavity about the cored liners, removing the
casting from the mold after the molten metal solidifies and then removing
the bonded cores from inside the cast in-situ ceramic liner by suitable
mechanical or chemical means. A product, such as a cylinder head or
exhaust manifold, is thereby cast with one or more ceramic liners cast
in-situ therein.
When this approach has been applied to the manufacture of cylinder heads
having cast in-situ ceramic exhaust port liners, the ceramic liners have
been observed to be susceptible to cracking during the casting process as
a result of differential thermal expansion differences between the ceramic
liner and the bonded core formed therein. In particular, when molten metal
is cast about the cored exhaust port liner, the liner and core can be
heated to a sufficiently high temperature that tensile stresses are
exerted on the liner by the bonded core which expands at a greater rate.
This thermal cracking problem is exacerbated by the brittleness, low
tensile strength and low thermal expansion coefficient of the ceramic
materials used to make the exhaust port liners.
The degree of susceptibility of the ceramic liner to cracking depends on
the configuration of the liner, the design of the cylinder head cast
therearound as well as the process used to cast the cylinder head.
Configurational and design modifications of the exhaust port liners can
sometimes be made to lessen or reduce their susceptability to cracking,
but these modifications typically increase the cost and the complexity of
the liners.
It is an object of the invention to provide a method of making cylinder
heads with ceramic exhaust port liners cast in-situ therein wherein
differential thermal expansion/contraction stress-induced breakage or
cracking of the ceramic exhaust port liners is reduced or minimized
regardless of the type (configuration) of the liners employed.
SUMMARY OF THE INVENTION
The invention contemplates a method of making an article, such as a
cylinder head, having one or more hollow, ceramic tubes, such as tubular
exhaust port liners, cast in-situ therein comprising (a) forming a
removable core in the ceramic tube of a core material having a thermal
expansion coefficient not exceeding 10 times the thermal expansion
coefficient of the ceramic tube whereby cracking of the ceramic tube from
differential thermal expansion-induced tensile stresses is minimized when
molten metal is cast around the ceramic tube and core, (b) casting molten
metal about the ceramic tube in an article-shaped mold cavity to form a
solidified, cast article with the ceramic tube cast in-situ therein, (c)
subjecting the cast article to an elevated temperature stress-relieving
treatment before the cast article cools to a lower temperature at which
harmful, crack-causing differential thermal contraction-induced
compressive stresses are exerted on the cast in-situ ceramic tube, and (d)
removing the core from the cast in-situ ceramic tube.
In one embodiment of the invention, the core comprises a shaped mixture of
the core material and a thermally decomposable binder, and the casting
mold comprises a shaped mixture of a mold material and a thermally
decomposable binder of the same or different type as the core binder. The
stress-relieving treatment is conducted with the cast article remaining in
the casting mold so that the core binder and the mold binder thermally
decompose to facilitate subsequent removal of the core material and the
mold material from the cast article.
A typical working embodiment of the invention involves making a cast iron
cylinder head having a plurality of ceramic exhaust port liners cast
in-situ therein. Preferably, the exhaust port liners comprise aluminum
titanate and the core formed in each aluminum titanate liner comprises a
cured/hardened mixture of zircon particulate and a thermally decomposable
resin binder. The zircon particulate core exhibits a thermal expansion
coefficient less than five times the thermal expansion coefficient of the
aluminum titanate liners to minimize cracking of the liners as a result of
differential thermal expansion stresses between the liners and cores when
the molten iron is cast therearound. After the molten iron is cast and
solidified around the cored liners, the cast cylinder head is stress
relieved in the casting mold at a temperature greater than about
1050.degree. F., preferably greater than about 1100.degree. F., for a
sufficient time to minimize cracking ceramic liners during subsequent
cooling of the cast cylinder head to ambient temperature. Typically, the
cast cylinder head is subjected to the stress-relieving treatment before
its temperature falls below about 500.degree. F.
The method of the invention permits the casting of articles, such as
cylinder heads, having one or more hollow ceramic tubes cast in-situ
therein with substantially reduced cracking or breakage of the ceramic
tube as a result of both tensile and compressive stresses exerted on the
ceramic tubes during the casting process. The method of the invention
lowers the susceptibility of various configurations of ceramic tubes to
cracking or breakage as they are cast in-situ in the article and renders a
wider range of configurations, preferably simpler, lower cost
configurations, of ceramic tubes usable in the manufacture of such
articles.
The invention may be better understood when considered in light of the
following detailed description of certain specific embodiments thereof
which is given hereafter in conjunction with the following drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a hollow exhaust port liner for use in
practicing the invention.
FIG. 2 is a perspective view of the exhaust port liner having a removable
sand core formed therein.
FIG. 3 is a schematic perspective view of a casting mold for use in
practicing the invention.
FIG. 4 is a fragmentary sectional view of the casting mold showing the
cored exhaust port liner positioned in the mold cavity.
FIG. 5 is a schematic perspective view showing molten metal being poured
into the casting mold.
FIG. 6 is a perspective view of a cast cylinder head made by practicing the
invention.
FIG. 7 is a fragmentary sectional view of the cast cylinder head showing
one exhaust port liner cast in-situ therein.
FIG. 8 is a view similar to FIG. 4 of another embodiment of the invention
where the exhaust port liner is coated with a ceramic layer.
BEST MODE OF PRACTICING THE INVENTION
Although the method of the invention is described in detail hereinbelow
with respect to making a cylinder head having a plurality of hollow
ceramic exhaust port liners cast in-situ therein, those skilled in the art
will appreciate that the method has wider applicability and is useful in
making myriad articles having one or more hollow ceramic tubes cast
in-situ therein.
The method of the present invention is illustrated in FIGS. 1 through 5 for
making the cast cylinder head 10 shown in FIGS. 6 through 7. The cylinder
head 10 comprises a cast iron, aluminum or other metal body 12 having a
plurality (four shown) of concave cylinder domes 14 cast therein with each
dome 14 including an inlet port 16 and an outlet (exhaust) port 18. Each
inlet port 16 extends through the cast body 12 to an intake
manifold-engaging surface 20. Each exhaust port 18 extends through the
cast body 12 to an exhaust manifold-engaging surface 22 where the usual
exhaust manifold (not shown) is attached.
A hollow ceramic exhaust port liner 24 is cast in-situ in the body 12 to
form and extend through each exhaust port 18. A typical hollow ceramic
exhaust port liner 24 is shown in FIG. 1 prior to being cast in-situ in
the cylinder head 10. Each liner 24 may include a slot 24a to accommodate
exhaust valving of the cylinder head.
The exhaust port liners 24 are fabricated from a suitable ceramic material,
such as cordierite, alumina, zirconia, aluminum titanate, beta-spodumene
and fused silica, which exhibits low density, low specific heat and a low
thermal conductivity. Of these, aluminum titanate is most preferred as a
result of its relatively high strength in compression and relatively high
elastic modulus that together yield a high strain to failure in
compression for this particular material.
The exhaust port liners 24 are shown in FIGS. 1, 2, 4 and 7 as having an
approximate circular cross-section and a gradual bend so as to extend
through the exhaust port 18 to the exhaust manifold-engaging surface 22.
The cross-sectional shape and bend of the exhaust port liners 24 can be
varied to accommodate different cylinder head designs as well as to lessen
stresses imposed on the liners 24 during manufacture of the cylinder head.
In accordance with the method of the invention for manufacturing the
cylinder head 10, each exhaust port liner 24 is first filled with a
mixture of ceramic core material and a thermally decomposable binder which
is curable and/or hardenable to form a bonded core 30 in each liner 24.
Typically, the mixture of core material and binder is cured and/or
hardened in each liner 24 in a core mold (not shown) to form shaped core
ends 30a,30b exposed outside each liner 24. As is known, the shaped core
ends 30a,30b are used to support the exhaust port liners 24 in desired
position in the mold cavity 40 of a casting mold 42, FIGS. 3 and 4.
Importantly, the core material is selected to exhibit a thermal expansion
coefficient that does not exceed 10 times, preferably does not exceed 5
times, the thermal expansion coefficient of the ceramic exhaust port
liners 24 such that thermal stresses exerted on the liners 24 are
maintained at a level to minimize breakage or cracking of the liners when
the liners 24 and their cores 30 are heated during casting of the cylinder
head 10 therearound in the casting mold 42.
For purposes of illustration only, when the exhaust port liners 24 are made
of the preferred ceramic material, i.e., aluminum titanate having a
thermal expansion coefficient of 1.0 micro inch/inch/.degree. C., the core
material will comprise zircon particulate (preferably 140 mesh size)
having a thermal expansion coefficient of 4.0 micro inch/inch/.degree. C.
Although the zircon particulate of the cores 30 has a higher thermal
expansion coefficient than that of the aluminum titanate liners 24, the
difference between the thermal expansion coefficient is small enough that
differential thermal expansion stresses exerted on the liners 24 during
casting (where the liners 24 and the cores 30 reach temperatures on the
order of 600.degree. F. in casting aluminum and 1200.degree. F. in casting
nodular iron) are insufficient to cause stress-induced cracks or breakage
of the liners 24 during that phase (casting phase) of the manufacturing
process. Thus, the thermal expansion coefficient of the ceramic cores 30
is regarded as being compatible with that of the ceramic liners 24.
As mentioned hereinabove, the ceramic core material is mixed with a resin
or other binder that cures and/or hardens to bond the zircon particulate
into the desired core shape. Preferably, the binder is selected to be
thermally decomposable at a stress-relieving temperature to which the cast
cylinder head 10 is subsequently subjected as explained hereinbelow. Since
the stress-relieving temperature used will depend on the particular metal
60 used to cast the cylinder head 10, the particular resin binder employed
will also depend on the metal being cast.
For purposes of illustration, when the core material comprises zircon
particulate and the molten metal 60 being cast is nodular iron, the resin
binder will comprise an organic oil-urethane resin curable in air over a
given time period or an organic phenolic/urethane/amine resin (catalyzed
by a catalyzing gas drawn through the mixture), both of which thermally
decompose at temperatures above about 600.degree. F. When the molten metal
being cast is aluminum using the same core material (zircon particulate)
for cores 30, the resin binder will comprise an organic polyol/urethane
(which sets or cures in air) that thermally decomposes at a temperature
above about 400.degree. F. By thermally decomposes it is meant that the
binder is vaporized or otherwise loses its capability to securely bond the
zircon particles together.
The oil/urethane binder for use in casting iron is a no-bake type
comprising an alkyd resin, polymeric isocyanate and lead salt catalyst.
The phenolic/urethane/amine binder is a cold box type comprising phenolic
resin, polymeric isocyanate and tertiary amine (gas). The polyol/urethane
binder for use in casting aluminum is a no-bake type comprising polyether
polyol and polymeric isocyanate.
Other organic or inorganic binders of the known cold box, hot box and
no-bake types may be used in practicing the invention, although organic
binders are preferred as a result of their thermal decomposability. These
known binder types are set forth in a technical article entitled "Updating
Resin Binder Processes", Parts I-IX published in Foundry m & T, 1986, and
authored by P. Carey and G. Sturtz.
As shown best in FIG. 4, each cored liner 50 (i.e., exhaust port liner 24
with the removable, thermal decomposable cores 30 formed therein) is
positioned in the mold cavity 40 of the casting mold 42 in usual fashion
with the core ends 30a,30b functioning as locators and supports for each
cored liner 50 in the mold cavity. The mold cavity 40 is configured in the
shape of the cylinder head 10 to be cast and may include other cores (not
shown) positioned therein to form coolant passages, oil passages and the
like typically present in the cast cylinder head 10.
Once the cored liners 50 and the other cores (not shown) are properly
positioned in the mold cavity 40, the mold cope 44 and the mold drag 46
are engaged, FIG. 5. The cope 44 and drag 46 are made of a mixture of
foundry sand (silica sand) and a thermally decomposable binder, preferably
the same binder as used in the cores 30. Molten metal 60 is poured into
the mold cavity 40 about the cored liners 50 and other cores (not shown)
therein. The molten metal 60 solidifies in the mold cavity 40 to form the
cast cylinder head 10 having the cored liners 50 and other cores cast
in-situ therein.
In accordance with the method of the invention, the cylinder head 10 is
subjected to a stress-relieving treatment after the molten metal
solidifies but before it can cool to a lower temperature at which harmful,
cracking-causing compressive stresses are exerted on the cast in-situ
cored liners 50 as a result of differential thermal contraction of the
cooling metal around the cored liners 50. When the cylinder head 10 is
cast of nodular iron, the cast cylinder head must not be allowed to cool
below a temperature of about 500.degree. F. before stress relieving the
cast cylinder head 10. On the other hand, for a cast aluminum cylinder
head, the cylinder head must not be allowed to cool below about
300.degree. F. without subjecting the cylinder head to the
stress-relieving treatment.
The stress-relieving treatment for a cast nodular iron cylinder head
involves subjecting the cylinder head to a temperature of greater than
about 1050.degree. F., preferably greater than about 1100.degree. F., for
a sufficient time to reduce internal stresses in the cylinder head to
avoid differential thermal contraction cracking-causing stresses on the
liners 24 during subsequent cooling to ambient temperature. For a cast
aluminum cylinder head, a stress-relieving temperature greater than about
575.degree. F., preferably greater than about 600.degree. F., has been
used. Times on the order of four hours have proved adequate for stress
relieving cast nodular iron and aluminum cylinder heads having the
aforementioned cored aluminum titanate liners 50 cast in-situ therein.
Preferably, the cast cylinder head 10 is stress relieved in the casting
mold 42 not only to stress relieve the cast cylinder head but also to
thermally decompose the binder of the cores 30 and the cope 44 and drag 46
to facilitate subsequent removal of the core material and mold material
from the cast cylinder head 10. The stress-relieving treatment is effected
by placing the casting mold 42 in a suitable furnace (not shown) before
the cylinder head cools below the critical temperature (500.degree. F. for
iron and 300.degree. F. for aluminum) and heating the cylinder head to
subject it to the desired stress-relieving temperature for the desired
time.
Following the stress-relieving treatment, the cast cylinder head 10 is
cooled to ambient temperature outside the furnace. Then, the cylinder head
is removed from the casting mold 42 and the thermally decomposed cores 30
are removed from the cast in-situ liners 24 by mechanical means, such as
blasting air.
Since the core binder is thermally decomposed during the stress-relieving
treatment, the cores 30 are easily removed and cleaned out of the cast
in-situ liners 24. Likewise, the cope 44 and drag 46 are also easily
removed from the cast cylinder head as a result of the mold binder being
thermally decomposed during the stress-relieving treatment.
The cast cylinder head 10 is then subjected to final machining operations
prior to assembly with other cylinder head components such as valves,
valve guides, cam shafts, etc.
The method of the invention envisions coating the exterior surfaces of the
liners 24 with a ceramic coating 70, FIG. 8, to further reduce
differential thermal contraction stresses exerted on the liners during
cooling of the cylinder head 10 cast therearound. The liners 24 are
typically coated by dipping in a slurry of a mica-based ceramic or
graphite. Coating thicknesses between about 0.5 mm and 2 mm have been
used. The mica-based coating or graphite coating both have higher strain
to failure in compression than the aforementioned aluminum titanate liners
24. As a result, the coating 70 is compressed by the differential thermal
contraction forces exerted by the cooling cylinder head 10 around the cast
in-situ cored liners 50 and thereby reduces compressive stresses exerted
on the liners 24 as the cylinder head 10 cools.
The method of the invention provides an economical, low cost method for
casting cylinder heads with cast in-situ ceramic exhaust port liners with
much reduced cracking or breakage of the liners during the casting
process. Moreover, the cores 30 in the exhaust port liners 24 and the mold
42 are readily removed following casting by virtue of the core binder and
mold binder being thermally decomposed during the stress-relieving
treatment carried in the casting mold 42.
While the invention has been described in terms of specific preferred
embodiments thereof, it is not intended to be limited thereto but rather
only to the extent set forth hereafter in the appended claims.
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